The constantly increasing operational speeds of digital computers are creating a demand for corresponding increases in the data storage capacities of magnetic tape recording and reproducing systems, while maintaining the special requirements of high speed digital tape systems.
It is desirable that tape recording and reproducing systems for use as computer data storage devices provide high data transfer rates and perform a read check on all written data. Toward those ends, conventional tape systems typically employ methods of recording known as linear recording, as illustrated in FIG. 1a, in which the tracks of data lie parallel to each other and to the edges of the tape; or helical scan recording, as illustrated in FIG. 1b, in which the tracks of data lie parallel to each other but diagonal to the edges of the tape. The linear recording method offers higher data transfer rates because of the ability to record and read multiple tracks simultaneously. As such, the number of tracks that are simultaneously written and read is referred to as a "band". The number of tracks within a band may differ, depending on the number of read/write gaps provided by the multiple channel head. It would be desirable to obtain higher data densities while retaining the advantages of linear recording.
One limitation on track densities is caused by crosstalk, which occurs when reading off one track is interfered with by data of an adjacent track. Crosstalk is further exacerbated by error in head gap alignments. Some methods have been implemented to reduce this effect, such as leaving guard bands between tracks, or by using wider write head gaps. These methods, however, also tend to limit track densities.
A method of recording known as azimuth recording has been used in helical scan systems in order to decrease the effects of crosstalk and thus increase the track density of these systems. Azimuth recording results in a recorded track pattern in which the magnetization directions of adjacent data tracks lie at different azimuth angles to each other as illustrated in FIG. 1c. This method greatly reduces intertrack crosstalk, allowing tracks to be placed closer together. The need for guard bands or wide write heads is thus reduced or eliminated.
One difficulty of high-density multiple channel magnetic recording, that affects all of the above-mentioned recording methods, is lateral tape motion (LTM). LTM is the random and unavoidable tendency for a tape to drift in a direction lateral to the direction of tape motion. During a tape write, lateral tape motion causes track directions to deviate from the parallel to the edge of the tape. During a read, lateral tape motion causes misregistration of the read head over the track being read. This misregistration results in read data error. Further error can be introduced by lateral motion of the write head during writing. To compensate for LTM, servo tracking techniques have been developed to reduce the effects of tracking error and thus improve the data capacity of tape systems. Known servo techniques vary widely, but most involve methods of dynamically moving the read head laterally to continually re-position it over the written data track. The movement of the read head gap compensates for lateral tape motion during a read.
Another difficulty of high-density multiple channel magnetic recording, that also affects all of the above-mentioned recording methods, is misregistration caused by changes in the environmental conditions within a tape drive. Specifically, changes in temperature, humidity and tension within the tape drive environment and stresses developed within the tape pack may cause lateral dimensional changes to the tape, i.e. tape width expansion and contraction. In addition, changes in environmental conditions (e.g. temperature and humidity) may also cause dimensional changes to the read/write head. Unlike LTM, where track pitch remains unchanged, tape expansion/contraction changes the spacing between longitudinal tracks (i.e. track pitch) as well as the relative spacing between the read/write gaps and the longitudinal tracks. If the dimensional change to tape width exceeds a certain percentage of the track width, simultaneous reading of the tracks within a band becomes impossible i.e. the signal strength from a misregistered track becomes undetectable. Particularly, as track density increases (e.g. 500 tracks on a 1/2 inch wide tape) and track width decreases (e.g. 0.2-0.3 mil read track width and 10-20 channel recording heads), misregistration becomes a greater problem. Prior approaches attempted to simply limit the ratio of the head width to the lateral span of the head. This limits either the track width or the number of tracks that can be utilized. Additionally, adjusting the lateral position of the read/write head would not re-align the read gaps with the data tracks because of the changing track pitch.
Therefore, there remains an unsolved need for a magnetic tape recording method that compensates track misregistration caused by track pitch changes.